Such a brake pressure control device is known from U.S. Pat. No. 4,080,005. In this reference, the key valve is an operating piston for a sensing arm, constructed as a slide piston, which will only in its position for an unloaded vehicle interrupt an otherwise open connection from the exit connection of the brake control valve, transmitting the first brake pressure to the brake cylinder and, for this purpose, transmits the first brake pressure to an air container. Simultaneously, the first brake pressure is transmitted to a separate valve organ containing the differential piston, which reduces it to the second brake pressure. By way of pipeline connecting the valve organ to the brake cylinder, the second brake pressure reaches the brake cylinder. Due to the division into a key valve and a separate valve organ, the known installation is costly to build and needs many connecting lines, which increase the costs of assembly. The slide valve in the key valve is a component that is subject to heavy wear, and the sensing arm is pressed during each braking action against an unsprung part for the whole duration of the braking in a loaded as well as in an unloaded vehicle, with the result that a relatively high degree of wear also is likely at the respective contact sites. In addition, the riding shocks acting on the sensing arm stress all the movable parts of the key valve and lead to unnecessary wear.
U.S. Pat. No. 3,671,086 discloses a non-generic but similar load-dependent brake pressure control device in which the sealing plate of the reduction valve can be intercepted by means of a stop on the differential piston and lifted by the latter from its valve seat. Otherwise, this arrangement shows defects similar to those cited above.
U.S. Pat. No. 3,612,621 discloses another load-dependent brake pressure control device in which the key valve is jointly constructed with the valve organ containing the differential piston and the reduction valve. However, this non-generic brake pressure control device requires a separate connection to a compressed air supply device.